The invention relates to a pressure-resistant process window for in-process, monitoring for visual or spectroscopic investigation of pressurized products in pipes and reactors. The process window consists at least of a measurement-cell body connected to the pipe or the reactor, and a transparent window pane, a seal between the measurement cell body and a window pane for sealing the reactor or pipe interior off from the environment, where the window pane is held against the measurement-cell body in a sealing manner by means of a screw barrel with an external thread which can be screwed into a hollow barrel with internal thread which is connected to the measurement-cell body.
Chemical production processes can be controlled efficiently if the current composition or quality of the product or of a reaction mixture is known at various stages of the production process. The requisite quality-relevant parameters can be determined with the aid of continuous on-line methods.
A particular key position in the determination of these parameters is held by spectroscopic methods, since these are able to determine current product properties without complex modification or treatment of the product being necessary.
The usual spectroscopic methods in this connection are UV/VIS spectroscopy (measurement of the absorption of the product in the wavelength range xcex=200-800 nm), NIR spectroscopy (measurement of the absorption of the product in the wavelength range xcexd=800-2500 nm), IR spectroscopy (measurement of the absorption of the product in the wave number range xcexd=4000-400 cmxe2x88x921) and fluorescence and Raman spectroscopy (excitation of fluorescence or Raman radiation by means of intense light sources).
An essential prerequisite for the usability of these methods is access to the process or product to be examined. Chemical processes are generally carried out in reaction tanks or pipes, which only allow spectroscopic measurement of the product therein after installation of windows which are transparent to the spectroscopic analysis radiation.
Due to possible toxic properties of the products to be analyzed in the process, high demands must be made of the reliability of process windows with respect to the leaks. This also applies in particular if processes are carried out under high pressure or high temperature.
Process windows of this type are available, for example, as inspection glasses. Inspection glasses are designed principally for visual monitoring of the reactor or pipe contents, less for spectroscopic process applications, in which a layer thickness, adjustable in a defined manner, of a product through which radiation is to be passed is required so that the desired product information can be derived from the spectrum.
Process windows for spectroscopic applications as part of an in-line measurement cell are described in the brochure from Optec-Danulat GmbH, D-45143, Essen, In-line photometry system overview 4.O, page 9 (corresponding to German Utility Model G 87 17 609.2). In these, stepped windows made of Pyrex or sapphire are pressed by means of a window ring against the window in contact with the product, which is sealed off from the product to be analyzed by means of an O-ring. The window ring is screwed to the measurement cell 1 by means of four screws. The pressure resistance of this known process window is essentially determined by the tensile strength and stripping force of the four clamping screws. The pressure resistance of this process window is inadequate for many potential applications.
A further process window is from U.S. Pat. No. 4,910,403. Here, a diamond window is soldered to a support which can be screwed in. This is designed, in particular, to be screwed into a standard pressure sensor port of an extruder. The typical diameter of the diamond window is 4.25 mm. If higher optical throughput is to be facilitated, the diameter of the diamond window has to be increased, which can result in very high material costs. A further disadvantage is the higher number of sealing surfaces required.
A sapphire window which has been soldered into a support is described analogously in the patent U.S. Pat. No. 5,151,474 of The Dow Chemical Company. Here too, it is problematic to implement greater diameters of the sapphire window in order to achieve higher optical throughput, since the soldering process is ever more difficult to carry out with increasing diameter. The unsatisfactory chemicals resistance of the solder to aggressive acids or lyes can also make the use of this technique for monitoring chemical processes impossible.
The object of the invention was to develop a process window which does not have the design disadvantages of the known arrangements, and in particular exhibits high pressure resistance and freedom from leaks over an extended operating period.
The object is achieved by a pressure-resistant process window for visual or spectroscopic,examination of pressurized products in pipes and reactors, which is the subject-matter of the invention, consisting at least of a measurement-cell body connected to the pipe or the reactor, and a transparent window pane, a seal between the measurement-cell body and a window pane for sealing the reactor or pipe interior off from the environment, characterized in that the window pane is held against the measurement-cell body in a sealing manner by means of a screw barrel with an external thread which can be screwed into a hollow barrel with internal thread which is connected to the measurement-cell body.
A preferred embodiment of the process window is designed in such a way that the hollow barrel has an annular sealing surface on which the window pane lies in a pressure-tight manner.
A particularly pressure-stable embodiment of the process window is designed in such a way that the hollow barrel is formed in one piece with the measurement-cell body or is welded thereto.
In a prefer variant of the process window, the hollow barrel is connected to the measurement-cell body in a pressure-resistant and detachable manner.
The process window may be designed in such a way that a ring of low friction to the barrel or the window pane is present between the screw barrel with external thread and the window pane and facilitates a pressure-resistant screw connection which is gentle on the window pane.
The window pane of the process window preferably has a greater wall thickness in the central region. This results in the formation of a bearing surface for, for example, ring seals. In addition, the reduction in the size of the side of the window pane in contact with the product achieves a reduction in the dead space (between the pipe wall and the window pane).
The ring between the screw barrel with external thread and the window pane preferably consists of graphite.
In a preferred variant of the process window, two rings which are in sliding contact with one another are present instead of a ring with low friction between the screw barrel with external thread and the window pane.
Examples of suitable materials for the transparent window pane are the materials which are basically known for the production of inspection glasses or spectroscopic windows, such as borosilicate glass, quartz or sapphire, which exhibit no or low absorption in the region of the typical relevant wavelengths mentioned at the outset for optical spectroscopy. In the case of glass, they can be produced in accordance with DIN 7080, 7081, 8902 and 8903 standards, so that they have official approval for the selected pressure range in accordance with the pressure container regulation (AD-N4). Such glasses are described, for example, in the brochure of xe2x80x9cTechnische Glaswerke Ilmenau GmbHxe2x80x9d, D 98684 Ilmenau.
The use of the process window is possible, without any claim to completeness, for the spectroscopic or visual determination of the chemical composition, substance identity, mixture analysis, purity of the substances flowing through the pipe or the reactor, and characteristic numbers, such as, for example, OH and acid numbers (NIR spectroscopy), for the determination of colour (VIS spectroscopy), for the determination of particle impurities or solids contents (scattered-light measurements) or for the visual inspection of the substances (with the eye or by means of a camera.
In the case of transmitted radiation measurement through a pipe, two of the windows according to the invention are installed opposite one another. It may then be necessary to increase the thickness of the installed window panes to the inside in order to reduce the absorption of the measurement radiation by the product.
Materials which are basically suitable for the window pane for the UV (200-400 nM), visible (400-800 nm), near infrared (800-2500 nm) and infrared (4000-400 cmxe2x88x921) spectral region are described in the book Bauelemente der Optik [Optical Components], 5th Edition by G. Schrxc3x6der, Hanser-Verlag 1987, ISBN 3-446-14960-0.
For use in the UV spectral region, emphasis should be placed on materials such as quartz, Suprasil quartz glass and sapphire, and the speciality glasses from Schott FK 5, UBK 7, UK 50 and BaK 2 (Schott Glaswerke, Hattenbergstraxcex2e 10, Mainz: optical glass, glass mouldings, radiation-protection glasses and windows).
Preferred window materials for use in the visible spectral region are sapphire, quartz, Pyrex glass and zirconium dioxide.
Preferred window materials for use in the near infrared spectral region are sapphire, zirconium dioxide and quartz.
Preferred window materials for use in the IR spectral region are zinc sulphide (ZnS), zinc selenide (ZnSe) and germanium (Ge).
Basically, all window materials are suitable which have adequately high transmission, strength, heat and chemicals resistance in the specific application.
The seals used for the process window can be either flat seals or O-ring seals.
The O-ring seal is preferably not located in an annular groove (for manufacturing au regions), but in a turned-out projection of the measurement-cell body. The depth of the projection is less than the diameter of the O-ring, and the width of the projection is greater than the diameter of the O-ring. By pressing the window against the O-ring or against the measurement-cell body with the aid of the screw barrel, a prestressing of the O-ring which is necessary for the correct sealing function is generated.
On use of flat seals, a groove is usually not provided. For fixing of the seal, a narrow and flat concentric groove or a narrow and flat turned-out projection can be worked into the measurement-cell body. The window is pressed against the seal with the aid of the screw barrel.
The choice of sealing material depends on the thermal and chemical load. In particular, use is made of elastic plastics, moulding compositions comprising inorganic fibres and binders, graphite and deformable metals, for example soft copper.
Proven flat-seal materials are:
PTFE (filled or unfilled, solid material or expanded material (Gore-Tex) with the advantage of very good chemicals resistance and heat resistance up to about 260xc2x0 C.
Graphite seals with metal inlay or without inlay (for example HDF-Flexitallic GmbH) with the advantage of heat resistance up to about 480xc2x0 C.
Proven seal materials are fluorinated elastomers, such as, for example, Kalrez(copyright) (manufacturer: DuPont de Nemour) and Viton(copyright) (manufacturer: DuPont de Nemour) and nitrile rubber or silicone rubber.
Instead of a sliding ring, it is also conceivable to use two rings which are in sliding contact with one another, in particular a so-called axial pressure bearing (ball or roller bearing).
The process windows according to the invention can also be installed in pairs opposite one another on a pipe.
This enables radiation to pass through a product in the pipe, for example for transmission measurements.
The invention furthermore relates to the use of the process window according to the invention for optical or spectroscopic process control, in particular of chemical reactions, and of mixing, conveying and separation processes.
The invention is explained in greater detail by way of example below with reference to the figures, without thereby restricting the invention in detail.